Large, flexible polymeric sheets, which are often referred to as membranes or panels, are used in the construction industry to cover flat or low-sloped roofs. These membranes provide protection to the roof from the environment particularly in the form of a waterproof barrier. As is known in the art, commercially popular membranes include thermoset membranes such as those including cured EPDM (i.e., ethylene-propylene-diene terpolymer rubber) or thermoplastics such as TPO (i.e., thermoplastic olefins).
These membranes, which may also be referred to as panels, are typically delivered to a construction site in a bundled roll, transferred to the roof, and then unrolled and positioned. The sheets are then affixed to the building structure by employing varying techniques such as mechanical fastening, ballasting, and/or adhesively adhering the membrane to the roof. The roof substrate to which the membrane is secured may be one of a variety of materials depending on the installation site and structural concerns. For example, the surface may be a concrete, metal, or wood deck, it may include insulation or recover board, and/or it may include an existing membrane.
In addition to securing the membrane to the roof—which mode of attachment primarily seeks to prevent wind uplift—the individual membrane panels, together with flashing and other accessories, are positioned and adjoined to achieve a waterproof barrier on the roof. Typically, the edges of adjoining panels are overlapped, and these overlapping portions are adjoined to one another through a number of methods depending upon the membrane materials and exterior conditions. One approach involves providing adhesives or adhesive tapes between the overlapping portions, thereby creating a water resistant seal.
Generally, there are two modes of membrane attachment that are used in conjunction to create a water impermeable roofing membrane assembly. The first seeks to anchor the membrane to the roof, while the second seeks to create a water impervious barrier by attaching individual adjacent membrane panels to each other or to flashing. Inasmuch as these modes of membrane attachment seek entirely different goals, the mechanisms by which they operate are likewise highly distinct.
With respect to the former mode of attachment, which involves securing of the membrane to the roof, the use of adhesives allow for the formation of a fully-adhered roofing system. In other words, a majority, if not all, of the membrane panel is secured to the roof substrate, as opposed to mechanical attachment methods which can only achieve direct attachment in those locations where a mechanical fastener actually affixes the membrane.
When adhesively securing a membrane to a roof, such as in the formation of a fully-adhered system, there are a few common methods employed. The first is known as contact bonding whereby technicians coat both the membrane and the substrate with an adhesive, and then mate the membrane to the substrate while the adhesive is only partially set. Because the volatile components (e.g. solvent) of the adhesives are flashed off prior to mating, good early (green) bond strength is developed. The contact bonding method employs adhesives that may include volatile organic compounds (i.e. solvent-based adhesives). Water-based adhesives are used for contact bonding as well, but the water does not flash off quickly and therefore the use of water-based adhesives for contact bonding may be fraught with problems.
The second method is known as wet lamination, whereby technicians coat the substrate with an adhesive and then mate the membrane with the substrate while the adhesive is still wet. This method can therefore advantageously employ water-based adhesives due to the fact that the mating step can occur immediately after application of the solvent, which significantly reduces installation time. But, wet lamination can only be used when the substrate to which the membrane is being adhered is porous (e.g. wood, plywood, or OSB board), which allows the water to dissipate through the substrate.
A third mode of attachment is through the use of a pre-applied adhesive to the bottom surface of the membrane. In other words, prior to delivery of the membrane to the job site, an adhesive is applied to the bottom surface of the membrane. In order to allow the membrane to be rolled and shipped, a release paper or liner is applied to the surface of the adhesive. During installation of the membrane, the release liner is removed, thereby exposing the pressure-sensitive adhesive, and the membrane can then be secured to the roofing surface without the need for the application of additional adhesives.
As is known in the art, the pre-applied adhesive is applied to the surface of the membrane in the form of a hot-melt adhesive. For example, U.S. Publication No. 2004/0191508, which teaches peel and stick thermoplastic membranes, employs pressure-sensitive adhesive compositions comprising styrene-ethylene-butylene-styrene (SEBS), tackifying endblock resins such as cumarone-indene resin and tackifying midblock resins such as terpene resins. This publication also suggests other hot-melt adhesives such as butyl-based adhesives, EPDM-based adhesives, acrylic adhesives, styrene-butadiene adhesives, polyisobutylene adhesives, and ethylene vinyl acetate adhesives.
In view of the nature of the adhesives, peel and stick membranes have inherent limitations. Specifically, there are temperature windows that limit the minimum temperature at which the membranes can be installed on a roof surface. Also, there are maximum temperature limits on the roof surface that the adhesive can withstand while maintaining wind-uplift integrity. With respect to the latter, where the surface temperature on the roof nears the glass transition temperature of the adhesive, the adhesive strength offered by the pressure-sensitive adhesive is not maintained. As a result, peel-and-stick membranes have not gained wide acceptance in the industry. Moreover, the use of peel-and-stick membranes has been limited to use in conjunction with white membranes (e.g., white thermoplastic membranes) because the surface temperature of these membranes remains cooler when exposed to solar energy.
With respect to securing adjoining membranes, the use of pre-applied tapes is also known. For example, U.S. Publ. No. 2010/0024955 teaches membrane assemblies that carry pre-applied adhesive tapes over a pre-primed lap region. Also, U.S. Publ. No. 2004/0191588, which is discussed above with respect to peel-and-stick membranes, alludes to the desirability of a membrane assembly where all of the components are peel-and-stick, but then teaches an assembly where stronger adhesives or more moisture-resistant adhesives are used for seaming membranes while weaker, cheaper adhesives are used for the non-overlapping region (i.e., to adhere the membrane to the roof surface).